P-cresol is a colorless liquid or crystal with special odor, which is corrosive and toxic. It is insoluble in water and soluble in alkaline solution or organic solvent, such as toluene, ethanol and so on. P-cresol is an important fine chemical feedstock, which is widely used in many industries, such as antioxidant, dye, medicine, pesticide, spice and so on. O-cresol, also known as 2-cresol, is the important fine chemicals intermediate in synthesis of pesticide, medicine, dye, synthetic resin, spice and antioxidant and so on. M-cresol, also known as 3-cresol, meta-cresol or m-methylphenol, is used mainly as pesticide intermediates to produce insecticide such as fenitrothion, fenthion, MTMC, permethrin. M-cresol is also used as intermediates for producing color film, resin, plasticizer and spices. Coal tar phenol contains approximately 30% of phenol, from 10% to 13% range of o-cresol, from 14% to 18% range of m-cresol, from 9% to 12% range of p-cresol and from 13% to 15% range of xylenol. Natural product separation method was used in the traditional process for producing cresol, and the three isomers of cresol can be recycled.
Due to the limitation of resources and the complex process with numerous separating equipments in the traditional process, chemical synthesis processes for cresol are developed after years of efforts and exploration. Since the success of chemical synthesis, the production equipment for cresol preparation via natural product separation is closed one after another. Nearly ten kinds of chemical synthetic routes have been reported.
Toluene sulfonated alkali fusion method is the traditional synthetic production technology for cresol. Toluene is sulfonated into toluenesulfonic acid, and then sodium hydroxide is used to treat molten sulfonated bodies to obtain cresol sodium salt. Sodium salt is mixed with water, into which sulfur dioxide or sulfuric acid is introduced for acidification to obtain cresol. Composition and content of cresol isomer depend on the reaction conditions, in which p-cresol is the main product and sulfonating agent could select from sulfuric acid or chlorosulfonic acid. Usually, alkali is sulfonated by sulfuric acid at the reaction temperature of 110° C., the product composing from 5% to 8% range of o-cresol and m-cresol, from 84% to 86% range of p-cresol and the dimethyl phenol; when chlorosulfonic acid used to sulfonate alkali at the reaction temperature of 40° C., the product compose from 84% to 86% range of p-cresol, from 14% to 16% range of o-cresol, without m-cresol. This method is mature, simple and suitable for the production of p-cresol. But usage of a large amount of strong acid and alkali bring the corrosion to equipment and serious pollution to environment, and the intermittent production mode is suitable for small-scale production. At present, it is the main method for producing p-cresol in China.
Toluene chlorination-hydrolysis method is cresol mixture obtained from toluene by chlorination substitution in benzene ring and hydrolyzation. Firstly, toluene contacts with chlorine gas at 230° C. in reactor to obtain the mixture of three kinds of chlorotoluene on the Cu—Fe catalyst. Secondly, the mixture is hydrolyzed to obtain cresyl sodium salt mixture at 425° C. on the catalyst SiO2. The hydrolysis reaction is continuous. The cresyl sodium salt solution is acidified and neutralized to obtain cresol mixture, which is separated by distillation to obtain p-cresol, o-cresol and m-cresol. The rate of m-cresol, o-cresol to p-cresol is 1:2:1. The method causes environmental pollution and the low quality product with lots of by-products.
Phenol alkylation method is production of o-cresol from phenol using methanol as alkylating agent. In the liquid phase condition, phenol reacts with methanol via methylation to obtain o-cresol on the Al2O3 catalyst, at the temperature from 300° C. to 400° C. and the pressure from 1 MPa to 3 MPa. The product comprises from 43% to 51% range of o-cresol, from 17% to 36% range of m-cresol and from 17% to 36% range of p-cresol. But comparing to other synthetic routes of mixed cresol, the method is uncompetitive due to the harsh reaction condition and lots of impurities in product.
Isopropyltoluene method is that isopropyl toluene is converted into methyl isopropyl benzene hydroperoxide under the initiation of peroxide free radical of hydrogen, and then oxidized by oxygen in air. Similar to the oxidization process of cumene to phenol and acetone, the products are rich in m-cresol and p-cresol, with acetone as by-product. But the complexity of this method is much higher than the synthesis of phenol. The product obtained from this method comprises hardly any o-cresol with the rate of m-cresol to p-cresol is approximate 7:3. It is the main process route for the m-cresol synthesis in the world. The purity of the product obtained from this method is high. It is suitable for production on a large scale. There are disadvantages of extremely technique difficult, long process and expensive distillation cost.
Presently, many articles and patents focus on the alkylation of phenol with methanol on the metal oxide catalysts. The main product is o-cresol with dimethyl phenol as by-product, without the high value-added p-cresol in the product. The reports about methyl cresol production by alkylation of benzene with phenol on the molecular sieve catalyst are less. The reaction performance of phenol and methanol on Beta Zeolite had been reported in Chinese Journal of Catalysis, 1998, 19 (5): 423-427. The stability of the catalyst modified by magnesium, manganese and lanthanum was low. It had been reported in Chinese Journal of catalysis, 2001, 22 (6):545-549 that the HZSM-5 catalyst modified by P2O5, MgO and Sb2O3 could improve the selectivity of aromatic ether and reduce the selectivity of cresol and dimethyl phenol. O-cresol selectivity increases with the increase of oxide loading amount. The appropriate oxide modification could improve the selectivity of p-cresol. After phosphorus modification, the para-selectivity increased to 35.87% and the cresol selectivity decreased to 44.10%. Alkylation of phenol with methanol on the hydrogen-form zeolite catalyst was reported in Appl. Catal. A: Gen, 342 (2008) 40-48, J. Mol. Catal. A: Chem., 327 (2010) 63-72 and Catalysis Today 133-135 (2008) 720-728, in which a large amount of nitrogen used as dilution gas, without a value for industrial application due to the harsh reaction condition and low stability of the catalyst. At present, there is no industrial unit for cresol production via gas phase alkylation of phenol with methanol using the molecular sieve as the catalyst. In traditional industrial production process of cresol, the usage of strong acid and alkali bring the generation of industrial waste water, the serious pollution to environment and the corrosion to equipment. To develop an industrial technology of cresol production via gas phase alkylation of phenol by using molecular sieve as catalyst is extremely urgent. The present invention aims at providing a method of cresol synthesis by shape alkylation of phenol with methanol, in which phenol and methanol are used as feedstock, modified molecular sieve are used as catalyst, the selectivity of cresol reached 90%, the para-selectivity reached 58%, and the stability of catalyst is good. The catalyst is environmental friendly and non-corrosive to the equipment with a good stability and a broad prospect in industrial application.